Polyyne-Type Materials - Polyynes: Synthesis, Properties, and Applications - page 197

investigate properties of polyyne-type materials, crystalline polyyne materi-
als such a crystalline carbyne (chaoite) or polydiacetylene analogs must be
synthesized in quantity, otherwise polyyne chains in the materials must be
stabilized by regular chemical structures. Poly( p-phenylenebutadiynylene)
PpPB had been considered as a p -conjugated polymeric model for carbyne
[6], because it is an alternate copolymer of phenylene and butadiynylene
that is adequately stable under atmospheric conditions at room tempera-
ture. The butadiyne moiety in the polymer is always stabilized by the
adjacent phenylene moieties in both sides. Thus this section is focused on
synthesis, properties, and possible applications of this type of polymer,
poly(phenylene-alt-oligoethynylene) analogs including metal complexes

in the polymer structure.

10.3.1 P OLY (p- PHENYLENE-ALT-OLIGOETHYNYLENE ) S

Poly( p-phenylene-alt-oligoethynylene)s are alternate copolymers of 1,4-
phenylene and

, ! -oligoethynylene (olygoyne) whose chemical structure is

summarized in Chart 10.3 . Among the series of the linear conjugated poly-

¼

mers, PpPB (Chart 10.3, m 2) was first synthesized by a Russian group

from 1,4-diethynylbenzene with the Glaser method [6], which was alter-
natively prepared in the presence of a Cu þ catalyst under aerobic condi-

tions (the Hay method) [22] ( Scheme 10.1(A) ). Poly(p-phenyleneethynylene)

PpPE (m 1) was prepared from 1,4-diethynylbenzene and dibromobenzene

(or diiodobenzene) in the presence of a Pd complex-CuI catalyst under inert
atmospheric conditions [23] by Sonogashira-Hagihara coupling (Scheme
10.1(B)). Poly( p-phenyleneoctatetraynylene) PpPO (m

¼

4) was prepared
from 1,4-bis(triethylsilylbutadiynyl)benzene by a modified Hay coupling
according to the reported method [24]. Triyne polymer (m

¼

3) and the
longer analogs (m>4) have not been synthesized so far. Three kinds of
polymers obtained as a yellowish white powder were ill-characterized mate-
rials with semi-conducting properties and almost insoluble in any solvents.
The stability of the polyyne-type polymers can be examined by thermo-
gravimetry and differential thermal analysis (TG/DTA). From the DTA

¼

curves in Figure 10.5 , an exothermic peak is observed in all cases. The very

broad peak for PpPE is observed around at 350 C, while the sharp peak is
observed at 207 C for PpPB and at 138 C for PpPO, respectively. The
exothermic peak temperature should be related with thermal stability of the
polyyne-type polymers, thus it is concluded that the most stable is PpPE

CHART 10.3 Chemical structure of poly( p-phenylene-alt oligoethynylene)s.